doubango open source 3GPP IMS/RCS framework
 

"doubango is the world's most advanced open source 3GPP IMS/RCS framework for both embedded and desktop systems.
The framework is written in ANSI-C to ease portability and has been carefully designed to efficiently work on embedded systems with limited memory and low computing power and to be extremely portable."

http://www.doubango.org/

any developer working on it for N900... it seems very promising..